Unsaturated polyester resins compositions are well known in the industry. The compositions often include ethylenically unsaturated monomers such as styrene. The monomers copolymerize with the unsaturated polyester to cross-link the composition. The compositions also contain "low profile additives" (LPA's). The LPA's are thermoplastic polymers that act to prevent undesirable shrinkage as the composition is being molded into a thermoset article.
The composition also may contain a fourth component known as a compatible component. The compatible component does not react with the unsaturated polyester, and improves surface quality and mold flowability. The four component resinous system imparts improved surface smoothness when used with other known, conventional ingredients for low-profile resin systems used in making sheet molding compounds (SMC).
New applications for polyesters have arisen in which the resins would be required to function in high temperature/corrosive environments. These encompass automotive structural resins (under-the-hood with exposure to hot oil, radiator fluid, etc. at high temperature). They also include resins for FDA approval able to withstand high temperature and repeated exposure to alkaline media. Heat distortion temperature (HDT) determines the upper service temperature of a part. Because high temperatures are commonly encountered under-the-hood, high HDT is a prerequisite for these new applications.
A common problem with polyester resins used in structural applications is that significant shrinkage occurs during cure. This causes serious problems since even small amounts of shrinkage cause bolt holes to miss-align, cross-members to warp, etc. Consequently, a so-called "zero-shrink" requirement for structural resins has become necessary to ensure dimensional stability. Including the appropriate amount of thermoplastic in an SMC formulation prevents excessive shrinkage and has been used for many years in the industry. Unfortunately, to reach the so-called "zero shrink" requirement, so much thermoplastic must be employed that unacceptably low HDT results. This fact sharply limits the use of polyester and vinyl ester-based resins in under-the-hood applications.